In wireless communication terminals, such as mobile phones, modulating the supply voltage of the power amplifier (PA) in such a way as to follow the envelope of the transmitted signal allows the PA to be operated at a higher efficiency—reducing power consumption. This technique is known as envelope tracking (ET). The circuitry/component that modulates the supply voltage input to the PA in this type of system is called an ET modulator.
In most mobile applications (e.g., mobile phones), the operating supply voltage is provided by a battery which is, in turn, input to the ET modulator for modulation, with the modulated supply voltage input to the PA. In some applications, the desired PA supply voltage may be higher than the available battery voltage. Usually when this happens, the transmitter is operating at maximum output power. During this time, the ET modulator can only output a modulated supply voltage that is equal to (or less than) the battery voltage—resulting in the PA supply voltage being clipped. This clipping behavior creates a significant amount of distortion that degrades the transmitted signal—showing up as adjacent channel power rejection (ACPR) degradation and error vector magnitude EVM) degradation.
Accordingly, there are needed circuits, systems and methods to reduce clipping of the supply voltage output from the ET modulator and input to the PA, thereby improving performance of the transmitter.